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Discussion Starter #1
Background – drove about 200 miles to the beach this past weekend. Filled up on the way down and the car got pretty normal gas mileage on that tank – average of 21ish, only part on the highway. Coming back, filled up on the homestretch and the car averaged 20, all highway, which is 6-7 low compared to our norm. Then this week, it guzzled 14 gallons in 71 miles! I use the road trip app on my phone, so these are all calculated numbers – BUT, the dash reads that it’s getting normal mileage.

Filled it up today to see how much gas it would take and it proved not to be the gauge (took a ton more than it should have for 71 miles). Gas engine, runs as well as ever, doesn’t smell like it’s leaking even when parked in the garage, no CEL or messages, 72k on the clock.

Any thoughts here??
 

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Purple Moderator
99 ML430, 00 ML320, 05 E500 4matic Wagon
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If there is a leak downstream of the MAF the unmetered air just destroys MPG.

Also all that extra exhaust is not good for the cat convertors.

Look for leaks on the intake tubes (both sides). Also cracks in the air cleaner box. If nothing
is found visually then take it to a good shop and have it smoke tested.
 

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Discussion Starter #3
I've had that happen with my bimmers, but not this bad. Would also think a condition that rich would throw a CEL. Will def take a look, thanks for the input. Wonder why the dash comp doesn't recognize the mpg loss?
 

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Figured I would post outcome for any future readers...car was checked out by our local indy shop and they couldn't find a thing wrong. Charged us $75 for computer time which was fair IMO. Mileage returned to normal over the next tank and a half (another long highway trip) so the consensus is bad gas.

I called the Exxon station and of course they told me they test their gas for water, etc and it was fine. The thing that gets me is that it never threw a code, never smoked, ran completely normal which I find odd for fuel that bad. The other possibility I guess is that the gas was stolen. Don't know if it's possible to siphon these tanks, but it's always locked when not in our garage and there was no apparent tampering with the fuel door.

Oh well, could have been worse!
 
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